1. Field of the Invention
The present invention relates to a continuous thru-wire welding machine and, more particularly, to a welding machine using flexure pivots and a dead weight principle for accurately controlling the welding force.
2. Description of the Prior Art
Complex electronic assemblies include large numbers of closely spaced, miniature terminals which must be electrically interconnected. This is often achieved simply and conveniently by the use of a continuous insulated wire, typically nickel coated with teflon. As is known in the art, the continuous insulated wire is snaked to individual terminal locations without the need of removing the insulation prior to assembling the wire for welding. Rather, advantage is taken of the cold-flow properties of teflon.
Specifically, a pair of elongate conductive electrodes having adjacent ends are mounted for movement towards the terminal. Either the electrodes can be concentric with a dielectric therebetween or the electrodes can be positioned in coaxial, spaced relationship and movable toward and away from each other. Typically, one electrode is hollow and the wire is continuously fed therethrough. When a connection is desired, the electrodes are moved into contact with the terminal. Pressure is applied between the electrode and the terminal to break through the teflon insulation. Thereafter, an electrical pulse is applied to the electrodes, which pulse causes a diffusion bond to occur at the junction of the wire and the terminal. The electrodes are then positioned consecutively to all of the other common points in a given signal string.
During such a process, it is desirable and, in fact, necessary, to accurately control the force on the wire positioned between the terminal and the electrodes. That is, if the force is too low, one does not achieve cold-flow of the teflon insulation and a good bond cannot be achieved. If the force is too high, the electrode will damage the wire. The result is the necessity to control the force within narrow limits. In the past, welding machines have utilized complex and expensive means for varying the pressure between the welding electrodes and the terminals. Furthermore, these machines have proven incapable of providing a repeatable, accurately controllable welding force.
For example, existing electro/pneumatic welding machines typically operate by actuating an electrical solenoid valve that extends upper and lower electrodes through a cable drive system to perform the wire bonding to the terminal. The electrical signal actuates the valve to pressurize a piston connected to cables and pulleys and the opposing electrodes sandwich the terminal and wire between them at a fixed pressure, controlled by a regulator, at which time a microswitch actuates the power supply to bond the wire to the terminal. Such a system is generally reliable, but does have problems associated therewith, Air pressure regulators tend to drift or cycle above and below the adjusted setting. The air pistons have considerable friction which affects the accuracy of the applied force. Furthermore, this system is dependent upon a clean source of air pressure. Contaminated air tends to corrode regulators due to moisture and also carries particles which create leakage across internal seals.
Existing electro/mechanical welding machines typically utilize an electronic servo system tied to a cable/pulley drive that extends upper and lower electrodes. The force level at the moment of bonding is electrically adjusted using potentiometers. The use of air pressure is completely eliminated. Actuator friction is compensated for by the servo system and reaction time is almost instantaneous for consistent force application at the electrode/terminal interface. Such a system is described in U.S. Pat. No. 4,179,597. While it is generally satisfactory for providing a repeatable, accurately controllable welding force, the system is complex and expensive.
A typical mechanical welding machine has a fixed bottom electrode and a movable upper electrode which is lowered using a cable/pulley arrangement. At the time the terminal is locked between the two electrodes, a constant force negator spring is extended to maintain a constant force level to pierce the wire insulation and bond the wire to the terminal. The power supply is fired by closing a microswitch that is actuated after the negator has been uncoiled a fixed distance. Such a system is very simple, but has inherent problems. That is, springs do change loadwise after extensive cycling. Reaction time is slow when force levels change at the weld point. Furthermore, friction levels can be high.